1. Field of the Invention
This invention relates to an electroplating apparatus and method for the electrodeposition of a metal onto only one side of a substrate immersed in the electroplating bath and, more particularly, to the fabrication of x-ray masks used in the manufacture of electronic components wherein gold is electroplated onto only the chrome/gold plating base membrane area on a boron doped silicon substrate to form the desired circuit configuration of the mask.
2. Description of Related Art
The electroplating of metal onto one side of a substrate immersed in the electroplating bath is desired for many plating operations varying from tools to electronic components. For convenience, the following description will be directed to electronic component plating and, in particular, to the fabrication of x-ray masks used in electronic component fabrication.
Polymeric materials, such as photoresists, are widely used in the semiconductor industry to produce masks of all types. In mask making the photoresist is overlaid on a surface in which the desired image is to be formed, exposed to the desired image and developed so that the image formed in the photoresist can be replicated in the underlying surface. To achieve the extremely fine details in the resulting replicated image on the underlying surface, necessary to X-ray masks, the photoresist is exposed by electron beams (E-beams).
The general process used to fabricate such X-ray masks is well known. In general, the process requires a substrate consisting of a typically 2.5 micron thick membrane region etched into a boron doped silicon wafer 100 mm in diameter. The substrate is bonded to a dielectric support ring. This assembly forms a mask blank. Alignment windows can be created in the blank by coating the entire surface of the blank with polyimide and etching the silicon away from the areas where the alignment windows are to be created. Once the alignment windows are formed, the polyimide is removed from the central membrane area but left in the area over the alignment windows. The entire blank surface, including the polyimide layer, is then coated with thin layers of chrome and gold as a plating base for later processing and overcoated with approximately 8000 Angstroms of an E-beam sensitive resist.
The photoresist layer on the blank is then exposed in an E-beam lithography system after which the photoresist is developed to form openings therein. A heavy layer of gold, which serves as an absorber, is then electroplated into the openings formed in the photoresist by the developing step. The photoresist and any exposed chrome and gold plating base layers, not covered by the electroplated gold, are usually now stripped from the blank surface leaving the gold absorber deposits on the silicon and polyimide surfaces. This forms an X-ray mask which may now be used to expose a photoresist layer on a semiconductor wafer.
In the production of such masks, extreme accuracy is required for any errors or distortions appearing in the mask will result in the same errors or distortions being replicated in the final products made using the mask. The present invention sets forth a method and apparatus for creating masks having enhanced accuracy.
One of the most important and difficult problems in x-ray mask fabrication is the elimination of defects. Since these masks must have good x-ray and optical transmission in the clear areas of the mask, many different types of defects will degrade mask performance. The reduction or elimination of any defect and foreign material contributor is critical in producing a perfect mask.
When using a gold plating cell apparatus, the gold plating solution is preferably on both sides of the x-ray membrane substrate. This eliminates any pressure differential between the front and back of the mask which can distort the membrane substrate. It also eliminates distortions caused by clamping the substrate with an 0-ring seal that is required to prevent plating solution from contacting the back of the substrate as commonly used in fountain plating systems. Consequently, this insures quality stress-free images with no mask pattern distortion and eliminates membrane breakage.
The front side of the boron doped silicon membrane has a thin chrome/gold plating base that is sputter deposited; this is the side of the mask to be plated with a thick (5000 .ANG.) gold absorber layer. Since the plating solution surrounds the front and backside of the membrane substrate and the backside of the boron doped substrate is somewhat conductive in areas, gold attempts to plate on the back of the mask. Since there is no gold or chrome plating base film on the backside of the mask, the gold tries to plate in some areas and has very poor adhesion to the silicon surface. This results in a gold backside residue which is a foreign material defect contributor since it can be rubbed off easily and can migrate to the front side of the mask.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an apparatus for electroplating a metal onto only one side of a substrate.
It is another object of the present invention to provide an apparatus for electroplating a metal onto only one side of a substrate in the fabrication of x-ray masks used in the manufacture of electronic components wherein gold is electroplated onto a boron doped silicon substrate.
It is an additional object of the invention to provide a device for holding a substrate in an electroplating cell for plating on only one surface of the substrate in a non-continuous immersion or rack plating system.
A further object of the invention is to provide a method for electrodepositing a metal onto only one side of a substrate and, in particular, to a method for the fabrication of x-ray masks used in the manufacture of electronic components wherein gold is electrodeposited onto a plating base on a boron doped silicon substrate.
It is yet another object of the present invention to provide electroplated articles including semiconductor substrates and x-ray masks made using the apparatus and method of the invention and electronic components made using the mask of the invention.
Still other objects and advantages of the invention will in part be readily apparent from the description.